backfire on throttle when warm after idling...

[an_unusual_eye]

i think i'm facing over-influx of information. hoping that someone has experienced exactly what i'm facing and may intersperse some clarity into what i've "learned" with the search and print sources. z-therapy video is on the way as well... anyway:

4 screw SU carbs... recent rebuild and "professional adjustment"... runs well cold and warm, and runs fairly efficiently.

until. you get. stuck in traffic.

after a few minutes of stop and go (warm), i get smallish backfiring under acceleration, which clears up on its own after a few minutes at steady speed.

can't find exactly this issue related anywhere with a "cure".

any and all insight appreciated.

best ~ rob

[Bruce Palmer]

Pull the choke on and see if that solves the problem. If it does, then you're more than likely lean which can show up as a spit back through the carbs.....

[an_unusual_eye]

thank you bruce... a great place to start. it would be truly ironic if, after years of trying to get the car to stop running rich, we'd gotten her to the point of negatively opposite. typical... but ironic.

[an_unusual_eye]

sorry to come back to this so long after... but the first real use the car has had since the last post was today. after some city driving, the problem is back full force, and doesn't seem to be clearing up on its own after 20 miles or so on the highway. not terrible at steady throttle, but even the slightest pressure on the gas peddle gets spits and sputters and small backfires out the exhaust. choke seems to have no beneficial effect at all. ran fine for 100 miles or so before the stop and go traffic. any insight will be greatly appreciated... i'm poking it with a stick an an autozone parking lot as we speak. many miles to go. thanks ~ rob.

Edited October 12, 2013 by an_unusual_eye

[zKars]

Look for vacuum leaks next.

[an_unusual_eye]

i decided to limp it home (another 40 miles or so)... the symptoms persisted until i got back to the highway, and another couple miles at 70mph, then cleared up like somebody flipped a switch. i can almost guarantee that the car will be totally fine until the next time i get into an extended idle or stop and go situation. checking vacuum lines now... but most are fresh.

[Captain Obvious]

I'm having a hard time forming a mental picture of what the symptoms are...

You're saying that it runs fine when you first start it and it continues to run fine until you do some stop-n-go type of driving?

And then after the stop-n-go, you can get onto the highway and the problem goes away all by itself after a couple highway miles?

[madkaw]

What year?

What fuel pump?

Fuel pump been checked for flow- measured?

Fuel filters been changed?

" professional adjusted" by who?

Ignition type?

Mixture screw adjusted to how far out?

[olzed]

Only when hot.

Fuel vaporizing maybe??

[an_unusual_eye]

thanks guys... first, it occurred to me today that i've used the term backfiring incorrectly. this is popping from the exhaust rather than backfiring.

Captain Obvious, you have the symptoms exactly. we used the car for over 100 miles today with no issue at all. started and parked 4 times, local and highway mix, but no long back to back red lights or construction delays.

madkaw... 1971, stock OE mechanical pump, not yet checked for flow rate, fuel filter was fresh and changed again yesterday, adjusted by a local brit-car-guru, ignition is pertronix/flame thrower, mixture screw... sadly i won't be able to answer until november, as we're only where the z lives for one weekend each month, and she's in the rear view mirror for october.

olzed... NOT only when hot. if the car is in "problem state" when parked, the condition remains when she's restarted cold.

Edited October 13, 2013 by an_unusual_eye

[madkaw]

Check the fuel rate.

Take the mixture screws a turn out and see if that improves anything.

How do the plugs look? your desciption lends to a lean condition. Your Guru might get the adjustments right for sync, but not sure how close he can GUESS on the mixture. My first inclination would be to richen things up before I did anything else. Then I would suspect fuel pump-IF I knew my gas tank was clean and not the culprit(dirty clogging fuel lines)

[Captain Obvious]

NOT only when hot. if the car is in "problem state" when parked, the condition remains when she's restarted cold.

That should make it easier to find, right? Mixture screws don't do that.

Stop-n-go driving isn't a particularly high fuel consumption situation. You've got way more fuel flowing through the carb (per unit time) on the highway than you do during stop-n-go. And the temperature things is weird for fuel supply. I'm having a hard time coming up with any sort of fuel supply issue that's temperature related. And certainly nothing that "latches" into a problem state as you've described.

Besides, it's not mid-summer anymore. If it's got a temperature related issue at this time of year, what was it like this past summer?

I'm thinking electrical... What's in the distributor? Is that the Pertronix?